This invention relates generally to electrographic developer apparatus, and more particularly to an improved magnetic brush developer apparatus having a selectively energizable electromagnet for forming the brush nap bristles to effect development of an electrostatic image.
In the electrographic process for making reproductions of input information, an electrostatic charge pattern is formed on the surface of an insulating member in image-wise configuration corresponding to the information to be reproduced. The charge pattern is developed by applying developer material to such pattern to form a visible image. The visible developer material image is then either transferred to a receiver member and fixed to such member, or fixed to the insulating member itself. In commercial high speed electrographic equipment, a common mechanism for developing electrostatic charge patterns is a magnetic brush developer apparatus.
A typical magnetic brush developer apparatus includes one or more magnets located within an applicator member. The applicator member may rotate about fixed magnets, or the magnets may rotate within a fixed applicator member. Developer material comprises, for example, a mixture of finely divided pigmented thermoplastic marking particles (toner) held to the surface of ferromagnetic particles (carrier) by electrostatic charges created by triboelectrification. The carrier particles, with the attached toner particles, are held on the applicator member in a bristle-like formation by the magnetic fields of the magnets to form a brush nap. Of course, developer material of the type comprised solely of marking particles which exhibit magnetic properties (referred to as single component developer) is also suitable. The developer material is then brought into contact with the electrostatic charge pattern by brushing the nap bristles across the surface of the insulating member. When the developer material contacts the surface of the insulating member, the electrostatic attraction for the triboelectrically charged toner particles by the charge pattern on the insulating member overcomes the attraction of the carrier particles for the toner particles and the pattern is developed (see, for example, U.S. Pat. No. 3,703,395, issued Nov. 21, 1972 in the name of Drexler et al).
Most magnetic brush developer apparatus utilize a cylindrical applicator member located in fixed spatial relation to the surface of the insulating member so that the brush nap of developer material is maintained in contact with the surface. However, in certain instances, it is desirable to selectively prevent the brush nap from contacting such surface. For example, in electrographically making multicolor reproductions, a plurality of related electrostatic charge patterns, corresponding respectively to color separation images of desired information to be reproduced, are formed on sequentially spaced areas of the insulating member (or on separate insulating members respectively). The electrostatic charge patterns are developed with complementary colored toner by a plurality of independent magnetic brush developer apparatus respectively. It is desirable that a brush nap of a particular one of the independent developer apparatus contact and developer only its respective charge pattern to prevent toner particles from the brush naps of the other developer apparatus from intermixing with, or scraping the toner particles of the such developed pattern.
The operation of the plurality of developer apparatus to develop only respective charge patterns has heretofore been accomplished, for example, by selectively elevating the independent magnetic brush developer apparatus so that their brush naps contact only respective charge patterns to be developed on the insulating member (see, for example, U.S. Pat. No. 3,854,449, issued Dec. 17, 1974, in the name of Davidson). Another manner in which the plurality of developer apparatus are selectively controlled to develop only respective charge patterns includes a plurality of mechanical skieves associated with the plurality of developer apparatus respectively. The skieves are selectively moved relative to their associated brush naps to regulate the height of the brush naps so that such naps contact only respective charge patterns to be developed (see, for example, U.S. Pat. No. 3,570,453, issued Mar. 16, 1971, in the name of Nuzum). Such development control mechanisms complicate the magnetic brush developer apparatus construction and are subject to failure due to the contaminating nature of the toner particles. Furthermore, they require a period of time to establish a developer material brush nap suitable for charge pattern development. Such period limits the operational speed of the reproduction apparatus because sufficient time must be provided in the reproduction process to establish the respective brush naps.